Midrash sa Exodo 19:12: Otzar Midrashim, Mekhilta DeRabbi Yishmael, Sifra & Kohelet Rabbah

Opposition 14) And a time to gather stones - corresponding to And He led Israel through its midst, and He brought them to Mount Sinai and gathered them and stationed them around Mount Sinai, as it says "And you shall set bounds for the people round about" (Exodus 19:12), and Israel are called "stones" as it says "From there is the Shepherd, the Stone of Israel" (Genesis 49:24), "The stone despised by the builders" (Psalms 118:22), and they received the Torah which is life forever - for His kindness endures forever.

(Exodus 19:12) "And you shall 'bound' the people": I might think only at the east of the mountain, (where they were encamped). It is, therefore, written "roundabout." "saying": We are hereby apprised that they exhorted each other in this regard. "Take heed unto yourselves": this constitutes a negative commandment. "not to go up on the mountain": I might think that he may not go up, but he may touch it; it is, therefore, written "Whoever touches the mountain." I might think that he may not climb and that he may not touch it but he may ascend in a sedan-chair; it is, therefore, written "not to go up on the mountain" (in any manner). "Whoever touches the mountain shall be put to death": This is the punishment.

"When the ram's horn sounds": (lit.,) when the ram's horn "draws out" its sound, you may ascend the mountain. R. Yossi says: It is not a man's place that honors him, but he that honors his place. As long as the Shechinah was on the mountain — "Whoever touches the mountain shall be put to death." Once the Shechinah has left the mountain, all are permitted to ascend it.